Various filters for clarifying a fluid are presently developed and produced. Among them, cartridge-type filters (hereinafter abridged as filter cartridges) are widely used in the industrial field, for example, for removing suspended particles in industrial liquid materials, removing cakes flowing out of a cake filtering apparatus and clarifying industrial water.
Several kinds of structures of a filter cartridge have so far been proposed. The most typical one is a bobbin winder-type filter cartridge, which is a cylindrical filter cartridge prepared by winding a spun yarn as a filter material on a perforated cylinder in a twill form and then fluffing the spun yarn. This type has long been used due to inexpensiveness and easiness in production. Another type of structure includes a nonwoven-laminated type filter cartridge. This is a cylindrical filter cartridge prepared by winding several kinds of nonwovens such as a carding nonwoven stepwise and concentrically on a perforated cylinder. A recent advanced technique in a nonwoven production has allowed some of them to be put to practical use.
However, the above-mentioned filter cartridges have several defects. For example, in the bobbin winder-type filter cartridge for trapping particles by means of fluffs of fluffed spun yarns and also in gaps of the spun yarns, it is difficult to control the size and form of the fluffs and gaps. This limits size and amount of the particles that can be trapped.
Further, an end face of a conventional bobbin winder type filter cartridge stays in a state that spun yarns are, so to speak, arranged and therefore is inferior in smoothness. Also, the yarns have to be wound tightly in order to prevent the yarns from being untied, so that the end face is hardened as well, and bite thereof into an edge of the housing is deteriorated, which has resulted in an inferior sealing property.
Several measures have so far been taken in order to solve such defects. To thermally bond an end face of a filter is one method. This thermal bonding of the end face has avoided untying of the yarns in a filter cartridge, but this is mainly effective just for preventing the yarns from being untied, and problems still remain unsolved in respect of smoothness and a sealing property of the end face as described above.
JP-U 4-87717 discloses a method in which a synthetic resin film wound on both end faces in the periphery of the filter is used as a binder, in order to prevent a cushioning material from being peeled in filtering paints. However, this method is used merely to solve the problem of rolling up of a glass fiber nonwoven caused in winding spun yarns, and not to solve all the problems described above.
Another method is one in which a cushioning material is adhered to an end face of a filter. In this method, a binder of a low density polyethylene film, a low molecular weight polypropylene or an ethylene-vinyl acetate copolymer resin is used to adhere a cushioning material such as foamed polyethylene, foamed polypropylene or ethylene propylene rubber. This method can be used to solve the problems of smoothness and sealing property of the end face. However, addition of such new materials restricts the use conditions of the filter, for example, the temperature condition and the chemical resistance of three materials i.e., a filter material, a cushioning material and a binder material, and therefore the filter is difficult to use in every fields. In particular, as the binders are mostly inferior in heat resistance and chemical resistance, they have sometimes experienced peeling off depending on the use conditions. Further, many of binders for such uses usually contains low molecular weight components, and therefore are problematic when used for general industries as well as medicines and foods.
JP-B 8-29206 discloses a method in which a thermoplastic sheet is bonded to an end face of a filter element comprising a thermoplastic synthetic fiber by means of a hot plate. However, this method is used to bring out a maximum sealing property at an end part of the original filter element, and if the original filter element is short of a potential sealing property (for example, flexibility), the method has not necessarily been satisfactory.
In a filter in which a broad nonwoven is wound around a perforated cylinder in layers as shown in FIG. 1, a so-called nonwoven-laminated type filter cartridge, its performance depends on the kind of nonwovens used. A nonwoven is produced mostly by a method in which short fibers are confounded by means of a carding machine or an air laid machine and then subjecting them, if necessary, to heat treatment by means of a hot air heater or a heating roll, or a method in which a nonwoven is directly prepared, such as a melt blowing method and a spun bonding method. However, any machines used for producing nonwovens, such as a carding machine, an air laid machine, a hot air heater, a heating roll, a melt blowing machine and a spun bonding machine, may often cause, for example, unevenness of a mass per unit area of a nonwoven in a machine direction. Accordingly, a filter cartridge produced will be inferior in quality.
Further, Japanese Utility Model Publication No. 7767/1994 proposes a filter cartridge in which a filter material obtained by squashing a tape-shaped paper having porosity while twisting, thereby squeezing it to control a diameter thereof to about 3 mm is wound around a porous internal cylinder in a close twill. This method is advantageous in that a winding pitch can be gradually increased from the porous internal cylinder toward the outside. However, the filter material needs to be squashed and squeezed, so that particles are trapped primarily between the winding pitches of the filter material. Accordingly, it is less expected to trap particles by filter materials itself as is the case of a conventional bobbin winder type filter using spun yarns which traps particles by means of fluffs. This blocks the surface of the filter to shorten the filter life or brings about the poor liquid-permeability in a certain case.
Proposed as another method, JP-A 1-115423 proposes a filter in which strings obtained by slitting a cellulose spun bonded nonwoven into strips and passing them through narrow holes to twist them are wound around a bobbin having a lot of drilled pores. It is considered that this method shall make it possible to prepare a filter having a higher mechanical strength and being free of dissolution in water and elution of a binder, as compared with a conventional roll tissue filter prepared by winding tissue paper in a roll form, which is produced from α-cellulose prepared by refining a coniferous pulp. However, the cellulose spun bonded nonwoven used for this filter has a papery form and thus a too high rigidity, and is liable to swell in a liquid. Swelling may bring about various problems such as a decrease in a filter strength, a change in a filtering accuracy, a deterioration in liquid-permeability, a reduction in a filter life and the like. Adhesion at fiber intersections of the cellulose spun bonded nonwoven is mostly conducted by a certain chemical treatment. Such adhesion is often unsatisfactory, causing a change in a filtering accuracy or falling of fiber chips, so that a stable filtering performance is difficult to achieve.
Further, JP-A 4-45810 proposes a filter prepared by winding a slit nonwoven comprising composite fibers in which 10% by weight or more of structural fibers is split into 0.5 denier or less on a porous core cylinder so that the fiber density becomes 0.18 to 0.30 (g/cm3). It is supposed that use of this method makes it possible to trap fine particles contained in a liquid by means of fibers having a small fineness. However, a physical stress has to be applied by means of high pressure water in order to split the composite fibers, and it is difficult to split evenly them all over the nonwoven by processing with high pressure water. Further, heterogeneous splitting lowers a strength of the nonwoven in a certain case. This may reduce strength of the resulting filter, so that the filter is liable to be deformed during use or the void rate of the filter is varied, so that the liquid-permeability is reduced.
On the other hand, separately from the methods described above, it has been attempted to raise a filtering performance by providing the filter with a multilayer structure. For example, JP-U 4-131412, JP-U 4-131413 and JP-U 5-2715 disclose a method for preparing a cylindrical cartridge filter comprising several layers by using a nonwoven comprising extra fine fibers which are obtained by splitting splittable composite fibers. These layer structures are a structure comprising a nonwoven-wound layer comprising extra fine fibers and a spun yarn layer (JP-U 4-131412), a structure comprising a slit nonwoven-wound layer comprising extra fine fibers, a layer prepared by winding a slit nonwoven and a yarn in combination and a spun yarn-wound layer (JP-U 4-131413) and a structure comprising a slit nonwoven-wound layer comprising extra fine fibers and a slit nonwoven-wound layer having a fiber diameter twice or more large as that of the above layer (JP-U 5-2715). All these filters having a multilayer structure are expected to extend the filtering life longer than those comprising a single layer structure. However, the problem caused by using split fibers as described above has not yet been solved.
Further, JP-U 4-30007 discloses a filter element having a two-layer structure comprising a core having a lot of communicating holes, a pleated filter prepared by folding many times a surface filter material in the periphery thereof to make it endless and a bobbin winder filter in the periphery thereof. However, spun yarns are used in this filter element, and therefore the problem caused by using the spun yarns as described above has not yet been solved.
An object of the present invention is to provide a filter cartridge which is excellent in filtering performances such as a liquid-permeability, a filtering accuracy and a filter life and in which fallen filter materials and other particles are not mixed in a filtrate and which can solve the problems of a smoothness of an end face and an inferior sealing property caused by a shortage in flexibility in sealing.